It has been traditional to burn coal in the presence of excess air to generate heat that can be used directly or indirectly for the purpose of providing steam energy demands. This steam is subsequently used in industrial boilers to provide heat or in utility boilers to generate electricity. It is well known that the amount of excess air is carefully controlled, not only for combustion-zone purposes, but also to maximize the residence time and heat transfer coefficient in the convective zone of the furnace. Therefore, under these conditions, oxygen partial pressure is primarily maintained by controlling the excess air flow rate which at times can be beneficial in the combustion zone, but detrimental elsewhere within the furnace.
One type of furnace used for combusting pulverized coal comprises a furnace firebox having a primary air source and a secondary air source which provide at least a sufficient amount of air for the complete combustion of the coal. The pulverized coal is fed into the firebox suspended in the primary airstream. Typically, a nozzle having concentric cylindrical passageways is the means for injecting the primary and secondary airstreams. The primary airstream containing suspended pulverized coal is fed through the central passageway of the nozzle while the secondary airstream is injected through the surrounding, or annular, passageway.
When coal having an ash content of less than about 6 wt. % is burned in such fireboxes, the fireball is usually compact and relatively well defined. However, when the ash content of the coal increases above 7 wt. %, the fireball begins to spread out, becoming more diffuse as the ash content increases because the higher ash coals require more excess air to burn properly and also burn more slowly. As a result, less radiative heat is generated for the same amount of coal fed into the furnace leading to severe derating of the steam capacity of the furnace. Further, the higher ash content means increased slagging in the firebox and also increased fouling of the tubes in the convective superheater sections due to higher ash emissions of particles less than about 1 micron in size.
U.S. Pat. No. 2,865,344 discloses adding oxygen to recycled combustion gases as a replacement for some of the air injected into the furnace in order to decrease stack heat losses and to better control furnace temperatures.
U.S. Pat. No. 3,628,322 discloses delivering oxygen to a gas producer to gasify coal in order to produce fuel which is then delivered with more oxygen to a boiler furnace to generate steam.
U.S. Pat. No. 3,699,903 discloses a method in which oxygen is used alone, preferably in stoichiometric quantities, as the secondary or combustion gas, and with only enough primary air for fuel feed, adequate gas velocities, and fuel turbulence and distribution in the furnace firebox.
U.S. Pat. No. 4,052,138 discloses a method of firing coal-powered boilers to produce both heat to operate the boiler and also to produce a usable fuel gas. The method consists of burning pulverized coal in the presence of varying proportions of steam and oxygen. The boiler is over-fired with coal, but with an undersupply of oxygen, in order that the total available heat content of the coal is divided between heat released by the incomplete combustion of the coal and heat content contained in combustible flue gases.
U.S. Pat. No. 4,261,167 discloses gasifying a carbonaceous fuel at relatively high pressure and temperature by partial oxidation with oxygen to produce a fuel gas, the fuel gas from the partial oxidation reaction is expanded to a lower pressure in a turbine and, after removal of undesirable compounds, is supplied to a steam boiler for the production of power.
U.S. Pat. No. 4,329,932 discloses a method of burning fuel with lowered emission of nitrogen oxides which comprises feeding pulverized fuel to the main burner in a combustion furnace and additionally feeding pulverized fuel to the region of the furnace where the first fuel is about to conclude its combustion, using inert gas with or without a low oxygen content as a conveying fluid, while supplying oxygen or air to a region downstream of the region for fuel addition.
B. Ghosh, et al., Ind. Eng. Chem., 47, 117-121 (1955) discuss the effect of oxygen enrichment on the ignition time of pulverized coal.
Other literature articles relating to oxygen enrichment of pulverized coal include:
A. Ivernel, Proc. Combustion Inst. European Symp. 1973, 463-468;
B. Ghosh, et al., 6th Symposium on Combustion, 595-601 (1957);
R. P. Weight, J. Inst. Energy, 54, 176 (1981); and S. Bandyopadhyay, et al., Combustion and Flame, 18, 411-415 (1972).